This packet includes five days of activities to be used in a high school chemistry class. These activities should accompany a 2-3 week long unit on acid base chemistry.
Rainbow Connection Demonstration- Introduces acids and bases in a visual way. Students will observe indicators and their response to changes in pH. They will also observe how pH change can be a reversible process. This could be the first day beginning a unit a acid base chemistry.
Measuring pH of Household Chemicals-This two day activity should follow several days of instruction on acids and bases. The pH scale should also be introduced in terms of concentration of H+ and OH- ions. The second part of this experiment is dependent on the availability of computer interfaced probes. The procedure of this part should be expanded or supplemented accordingly. If probes are unavailable the second day may be omitted.
How Does a Buffer Affect pH?- This experiment should follow a discussion of buffer solutions in terms of H+ cations, OH- anions, and disassociation of weak and strong bases. Students should also be able to recognize the acid and base components of conjugate acid/base pairs.
Microscale Titration-This activity uses phenolphthalein to indicate when an acidic acid has been titrated with a NaOH solution. By knowing the normality of the NaOH solution and the amount of drops used to titrate the acidic acid solution, the normality of an acidic acid solution can be calculated. This experiment should follow several lectures on the normality of solutions. Students should also be familiar with different measures of concentrations including molarity and molality.
Teacher Support Materials:
Rainbow Connection Demonstration Overview:
This demonstration will be an introduction to a weeks worth of activities exploring acid/base chemistry. Several different ph indicators will be used to produce a wide array of colors. These indicators will be colorless under acidic conditions, but when a base is added, they will produce the three primary colors. By carefully mixing the indicators, all the colors of the rainbow will be produced. Six beakers are filled with the same “mystery” liquids. Each glass beaker takes on a different color of the rainbow, despite the fact that the same liquids were added to each. These solutions can be used several times and have a good shelf life.
Teacher and Student Objectives:
Shows the dramatic color change of solutions containing indicators at different pH levels.
Demonstrates the reversibility of indicator color changes.
Get students thinking about how H+ and OH- ions interact.
Who’s Being Taught?
This demonstration could be used in both junior and high school when covering acids bases and indicators. Class size can vary considerably. It could even be given to a large audience given the proportions are increase to increase visibility.
Strengths of Exercise:
This is a vivid exercise that should hold the student’s interest.
The idea that you are performing magic using ‘mystery’ solutions can be used to increase their interest.
This demonstration makes a great introduction to acid/base chemistry.
Easy disposal of chemicals.
How to Assess:
Assign the students the readings in their textbooks that begin with acid base chemistry. Be sure to include readings on pH indicators. Students then should have some understanding about how this lab was executed.
What to Look Out For:
Precautions should be taken when making solutions. (Safety Glasses)
Students will not be handling solutions so it should be safe if they don’t wear safety glasses. This would also depend on school policy.
Beakers should be visible to the entire audience.
Demonstration should be tested before presenting.
The solutions in this demonstration should be prepared at least a day before the presentation.
Some thought should be given on how this demonstration should be given.
The following recipes are for the six different indicator solutions. Each recipe will produce 15 mL of solution. These solutions should be made at least one day in advance to allow the solutes to fully dissolve into solution. Use 15 mL of 95% ethanol as the solvent for all six solutions.
Red- 0.4 g phenolphthalein plus 1.0 g m nitrophenol
Orange- 0.15 g phenolphthalein plus 2.0 g m nitrophenol
Yellow- 2.0 g m nitrophenol
Green- 0.2 g thymolphthalein plus 2.0 g m nitrophenol
Blue- 0.5 g thymolphthalein
Violet- 0.3 g phenolphthalein plus 0.13 g thymolphthalein
Mix 250 mL 0.05 M aqueous sulfuric acid with 250 mL 95% ethanol. Transfer into a large pitcher.
Prepare 1200 mL of 0.012 M NaOH solution transfer into a large pitcher. Make sure you know which pitcher is the acid and which is the base.
Prepare 100 mL of 0.2 M NaOH solution and transfer into a dropper bottle.
Dissolve 10 mL of 18 M H2SO4 in 20 mL of glycerol. Transfer into a dropper bottle.
Before Class: Arrange the six 250 mL beakers in a row so that they can be seen by the entire class. Carefully place 2 drops of indicator solution in the middle of the beaker in the order shown:
Let the indicator solution dry on the bottom of the glass. This should take about 30 minutes.